For example in electric vehicles, hybrid electric vehicles, and the like, a rechargeable battery (secondary battery) is used to provide power (discharge) to an electric motor used for driving such vehicles and to accumulate electric energy by charging from a ground-based power supply or from the electric motor, which is caused to function as an electrical generator of energy during braking.
In this case, in order to maintain the battery in an optimal state for an extended period of time, it is necessary to perform battery management by constantly monitoring the state of the battery, i.e. the State Of Charge (SOC).
As conventional battery state of charge detection methods, a sequential state recording (bookkeeping) method (also referred to as a current integration method or coulomb counting method) and an open circuit voltage estimation method are known. In the sequential state recording method, input and output of the battery voltage, current, and the like are recorded as time series data, the electric charge at the present time is determined by time integrating the current using these data, and the SOC is determined using the initial value of the electric charge in the charged battery and the full charge capacity. In the open circuit voltage estimation method, the input current value and terminal voltage value of the battery are input, and using a battery equivalent circuit model, the open circuit voltage value, which is the state quantity in this model, is sequentially estimated. The state of charge is then estimated based on this open circuit voltage value.
These methods have both advantages and disadvantages. The current integration method is more accurate than the open circuit voltage estimation method for estimating the state of charge over a short time, yet as the time lengthens, error is accumulated, causing accuracy to worsen. By contrast, the open circuit voltage estimation method does not require constant observation, yet since the open circuit voltage varies little with respect to change in the state of charge, this method is inferior to the current integration method when estimating the amount of variation in the state of charge over a short time.
To address this issue, an apparatus for state of charge estimation that improves the state of charge estimation accuracy by correcting the estimation error of the state of charge using both methods for state of charge estimation (i.e. a sensor fusion technique) is known.
A known example of such a conventional sensor fusion type apparatus for battery state of charge estimation includes a first calculation unit that calculates a first remaining capacity by time integrating a charge/discharge current of a battery using a current integration method, a second calculation unit that calculates a second remaining capacity from an open circuit voltage by estimating the open circuit voltage from impedance of a battery equivalent circuit model based on the charge/discharge current and a terminal voltage of the battery using an open circuit voltage estimation method, and a third calculation unit that calculates a remaining capacity of the battery by combining the first remaining capacity and the second remaining capacity after weighting with a weight that is set in accordance with usage conditions of the battery (for example, see Patent Literature 1).